1. Field of the Invention
The present invention relates to a screen for use in a projection display which displays an image by projecting it onto a screen.
2. Description of the Prior Art
As shown in FIG. 1, a projection display enlarges an image displayed on, e.g., a cathode-ray tube 11 and projects the enlarged image onto a screen 13 by using a projection lens 12. Such projection displays are divided into two categories, a front type (reflection type) and a rear type (transmission type), according to the type of the screen 13.
The screen 13 of the rear type projection display is, as shown in FIG. 2, generally constituted by a longitudinal-stripe lenticular lens screen 14, in which lenticular lenses extend in the longitudinal direction, and a Fresnel lens screen 15. Note that a mirror 16 is arranged between the projection lens 12 and the screen 13 to bend an optical path, thereby decreasing the dimensions of the projection display.
The Fresnel lens screen 15 causes light 17 of a projected image to be incident perpendicularly to the lenticular lens screen 14. The lenticular lens screen 14 refracts the incident light 17 and thereby diffuses the light 17 emerged from it.
In a conventional screen of a projection display, as shown in FIGS. 3A and 3B, a light diffusing agent 21 is mixed in the lenticular lens screen 14 in order to increase the diffusion angle of the light 17. Note that, in order to prevent a decrease in contrast caused by reflection of external light, a black surface 22 which consists of black stripes is formed on the exit surface of the lenticular lens screen 14 except for the positions of the lenticular lenses.
In a conventional screen of a projection display of the above sort, however, although a practically satisfactory angle of view can be obtained in the horizontal direction with the aid of the lenticular lens screen 14, the angle of view in the vertical direction is narrow since only the effect of the light diffusing agent 21 is useful in this direction. When the angle of view in the vertical direction is narrow, a screen which is bright when a viewer watches it while sitting in a chair or on a floor is dark if, for example, he or she watches it while lying on a floor, and so no high-quality images can be displayed in that case.
To widen the angle of view in the vertical direction, therefore, the amount of the light diffusing agent 21 to be mixed in the lenticular lens screen 14 may be increased, or, as shown in FIG. 4, a transverse-stripe lenticular lens screen 23 may be superposed on the longitudinal-stripe lenticular lens screen 14.
The use of the two lenticular lens screens 14 and 23, however, increases the manufacturing cost. For this reason, as shown in FIGS. 5A to 5C, it has been attempted to use a single microlens screen 24 in which a plurality of microlenses are arranged into a matrix-like pattern in place of the two lenticular lens screens 14 and 23.
If, however, the amount of the light diffusing agent 21 to be mixed in the lenticular lens screen 14 is increased, reflection of incident light is increased to decrease the light transmittance, and this decreases the brightness and contrast of an image.
In addition, if the transverse-stripe lenticular lens screen 23 is additionally formed and the light diffusing agent 21 is mixed in both of the two lenticular lens screens 14 and 23, the light transmittance decreases to decrease the brightness and contrast of an image. If, in contrast, the light diffusing agent 21 is mixed in neither the lenticular lens screen 14 nor 23, Moire becomes conspicuous between the lenticular lenses of the transverse-stripe lenticular lens screen 23 and scan lines of an image. Therefore, none of the above conventional examples can display high-quality images.
FIGS. 6 to 8 illustrate a three-tube rear projection display and a conventional example of a screen used in this display. In this display, cathode-ray tubes 31, 32 and 33 for displaying red, green and blue images are arranged horizontally, and projection lenses 41 to 43 project light components 34 to 36 which are modulated by their respective colors. The screen is constituted by a Fresnel lens screen 44 and a lenticular lens screen 45. Note that no mirror is shown in these drawings.
The Fresnel lens screen 44 causes the light components 34 to 36, which are projected by the projection lenses 41 to 43, to be incident perpendicularly to lenses 45a on the incident side of the lenticular lens screen 45. The lenticular lens screen 45 synthesizes the incident light components 34 to 36 to form a color image and also widens the light components 34 to 36 of a displayed image, which emerge from lenses 45b on the exit side, in the horizontal direction rather than in the vertical direction.
Since the cathode-ray tubes 31, 32 and 33 for displaying red, green and blue images are arranged at different positions, angles at which the light components 34 to 36 modulated by their respective colors are incident on the lenticular lens screen 45 also differ from one another. Consequently, as shown in FIG. 7, images of the light components 34 to 36 are formed at different positions on the surface of each lens 45b on the exit side.
Between the individual lenses 45b, on the other hand, a black nonreflecting portion, i.e., a black surface, which is called black stripes 46, is formed into longitudinal stripes in order to absorb external light and to thereby increase the contrast of a displayed image with respect to the external light. In this conventional example, the black stripes 46 project in the exit direction of the light components 34 to 36 farther than the lenses 45b.
In this conventional example as described above, if the screen is viewed obliquely, as shown in FIG. 8, all of the light components 34 to 36 of the respective colors shown in FIG. 7 can be seen in a region in which an angle defined between the normal to the lenticular lens screen 45 and the viewing direction of an eye is small (angle=.theta..sub.2).
In this conventional example, however, the black stripes 46 project in the exit direction farther than the lenses 45b as described above. Therefore, in a region in which the angle defined between the normal to the lenticular lens screen 45 and the viewing direction of an eye is large (angle=.theta..sub.1), it is impossible to see the light components 34 to 36 emerged from the skirt of the lens 45b since they are eclipsed by the black stripe 46.
This gives rise to a phenomenon called color shift or color shading in which, even if an image of the same color is displayed on the entire surface of the screen, the color of the image changes depending on the angle at which a viewer watches the screen. Therefore, the conventional example shown in FIGS. 6 to 8 cannot display high-quality images.